One of the Gothic wonders of the medieval world, the stained glass windows at Sainte-Chapelle in central Paris have been restored after seven years of painstaking work.

The restoration was finished to mark the 800th anniversary of the birth of King Louis IX, who commissioned the chapel in the mid 13th century to house his collection of religious relics, including what was believed to be Christ’s crown of thorns and part of the cross.

The work involved dismantling the huge windows into small panels and cleaning them with lasers. An outside “skin” of glass has been moulded on to the original windows to protect them from traffic pollution, without altering their look.

The chapel is one of the earliest surviving buildings of the Capetian royal palace on the Ile de la Cité, along with the Conciergerie.

The two-level building, which was built in just seven years in the 1240s, is a small but spectacular example of the Rayonnant style of Gothic architecture, with little stonework and 15 huge stained glass panels and a rose window added a century later.

The chapel suffered several fires and one flood, but it was most badly damaged during the French revolution when its spire was removed and the relics scattered. Some of them were retrieved and are now kept in nearby Notre Dame cathedral.

The Sainte-Chapelle was then converted into an administrative office and archives, and windows were removed to give more light. Those that remained were all but hidden by enormous filing cabinets which, in fact, helped preserve them.

Left to decay, and in a pitiful condition by the mid 19th century, the chapel underwent a restoration in 1836 based on descriptions and drawings of the original building.

Saint-Chapelle's gothic architecture is a miracle of light

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The royal palace around the chapel has long since disappeared and it is now surrounded by the Palais de Justice. The Conciergerie, where Marie Antoinette was held before being introduced to the guillotine, is next door.

The religiously devout Louis IX spent 40,000 livres building the chapel that houses the crown of thorns, bought for 135,000 livres from the Emperor of Constantinople. He also acquired a piece of “the true cross”.

The chapel, which now attracts more than 900,000 visitors a year, was visited by Adolf Hitler in June 1940 during his only trip to the French capital.